The invention relates to a compact time-of-flight mass spectrometer which enables very accurate mass determinations.
The best choice of mass spectrometer for measuring the mass of large molecules, as undertaken particularly in biochemistry, is a time-of-flight mass spectrometer because it does not suffer from the limited mass range of other mass spectrometers. Time-of-flight mass spectrometers are frequently abbreviated to TOF or TOF-MS.
Two different types of time-of-flight mass spectrometer have been developed. The first type comprises time-of-flight mass spectrometers for measuring ions which are generated in pulses in a tiny volume and accelerated axially into the flight path, for example with ionization by matrix-assisted laser desorption, MALDI for short, a method of ionization suitable for ionizing large molecules.
The second type comprises time-of-flight mass spectrometers for the continuous injection of an ion beam, one section of which is ejected as a pulse in a xe2x80x9cpulserxe2x80x9d transversely to the direction of injection and forced to fly through a mass spectrometer with reflector as a linearly spread ion beam lying transverse to the direction of flight, as the schematic in FIG. 1 shows. A ribbon-shaped ion beam is therefore generated in which ions of the same type, i.e. with the same mass-to-charge ratio, form a transverse front. This second type of time-of-flight mass spectrometer is known for short as an xe2x80x9cOrthogonal Time-of-Flight Mass Spectrometerxe2x80x9d (OTOF); it is mainly used in conjunction with out-of-vacuum ionization. The most frequently used type of ionization for this type of mass spectrometer is electrospray ionization (ESI). Electrospray ionization (ESI) is suitable for ionizing large molecules in much the same way as MALDI. It is also possible to use other types of ionization, for example chemical ionization at atmospheric pressure (APCI), photoionization at atmospheric pressure (APPI) or matrix-assisted laser desorption at atmospheric pressure (AP-MALDI). Ions generated in-vacuum can also be used. Before they enter the OTOF, the ions can also be selected and fragmented in appropriate devices so that the fragments can be used to improve the characterization of the substances.
In this second type of time-of-flight mass spectrometer, a large number of spectra, each with relatively low ion counts, are generated by a very high number of pulses per unit of time (up to 20,000 pulses per second) in order to utilize the ions of the continuous ion beam as effectively as possible.
As with all mass spectrometers, with a time-of-flight mass spectrometer one can only determine the ratio of the mass m of the ion to the number z of elementary charges which the ion carries. Any subsequent reference to xe2x80x9cspecific massxe2x80x9d or quite simply to xe2x80x9cmassxe2x80x9d on its own always means the ratio m/z. If, by way of exception, xe2x80x9cmassxe2x80x9d in the following text is to be taken to mean the physical dimension of the mass, it will be specifically called molecular mass The unit of molecular mass m is the xe2x80x9cunified atomic mass unitxe2x80x9d, abbreviated to xe2x80x9cuxe2x80x9d, usually simply termed xe2x80x9cmass unitxe2x80x9d or xe2x80x9catomic mass unitxe2x80x9d. In biochemistry and molecular biology, the unit xe2x80x9cDaltonxe2x80x9d (xe2x80x9cDaxe2x80x9d) is still frequently used. The unit of specific mass m/z is xe2x80x9catomic mass unit per elementary chargexe2x80x9d or xe2x80x9cDalton per elementary chargexe2x80x9d, where the elementary charge is the charge on an electron (if negative) or proton (if positive).
FIG. 1 shows the principle of a reflector time-of-flight mass spectrometer with orthogonal ion injection. In the pulser, the ions are accelerated transversely to their direction of injection (x-direction); the direction of acceleration is called the y-direction. The ions leave the pulser through slits in slit diaphragms, which can also be used for angular focusing in a z-direction which is at right angles to the x- and y-directions. After being accelerated, however, the ions have a direction which lies between the y-direction and the x-direction, since they fully retain their original velocity in the x-direction. The angle to the y-direction is xcex1=arctan √(Ex/Ey), where Ex is the kinetic energy of the ions in the primary beam in the x-direction and Ey the energy of the ions after being accelerated in the y-direction The direction in which the ions fly after the pulsed ejection is independent of the mass of the ions.
The ions which have left the pulser now form a broad ribbon, where ions of the same type (the same specific mass m/z) are all to be found in one front, which has the width of the beam in the pulser: Light ions fly faster, heavy ones slower, but all fly in the same direction, with the exception of possible slight differences in direction which can arise as a result of the slightly different kinetic energies Ex of the ions as they are injected into the pulser. These ions are therefore injected as monoenergetically as possible. The field-free flight path must be completely surrounded by the accelerating potential in order not to disturb the ions in flight.
As reported by W. C. Wiley and I. H. McLaren (Rev Sci Instrum 26 (1955) 1150), ions with the same specific mass which are at different locations of the beam cross section can be time-of-flight focused with respect to their different start locations by selecting the field in the pulser in such a way when switching on the outpulsing voltage that the ions furthest away are given a slightly higher acceleration energy to enable them to catch up with the leading ions again in a time-of-flight focal point. The time-of-flight focal point can be positioned as desired by means of the outpulse field strength in the pulser. This converts the initial spatial dispersion of the ions into an energy dispersion. The energy dispersion is compensated by the reflector in the known way.
To scan ion beams in time-of-flight spectrometers, instruments currently commercially available incorporate so-called channel plate secondary-electron multipliers by which the ion beams are amplified; these amplified currents are fed into fast transient recorders. The fast transient recorders digitize the amplified ion beams at the rate of one to four gigahertz in analog-to-digital converters with a signal resolution of usually eight bits.
In order to achieve a high resolution, the mass spectrometers (both axial and orthogonal time-of-flight mass spectrometers) are equipped with at least one energy focusing reflector which reflects the outpulsed ion beam toward the ion detector, thereby accurately time focusing ions of the same mass but slightly different initial kinetic energy in the y-direction onto the large-area detector. The ions fly out of the (last) reflector towards a detector which, in the case of orthogonal time-of-flight mass spectrometers, must be of the same width as the ion beam in order to be able to measure all incident ions. This detector also must be aligned parallel to the x-direction, as shown in FIG. 1, in order to also concurrently detect the front of flying ions of the same mass.
The resolution R and the mass accuracy of a time-of-flight mass spectrometer are proportional to the flight distance. It is therefore possible to increase the resolution by selecting a very long flight tube or by introducing several reflectors to produce multiple reflections. For example, with a flight path of one and a half meters one can achieve a mass resolution of around R=m/xcex94m=10,000; with around six meters, a mass resolution of R=m/xcex94m=40,000 (where xcex94m is the line width of the ion signal at half maximum, measured in mass units).
Flight tubes of several meters in length are very inconvenient because they result in unwieldy instruments. Multiple reflections are also problematic, however, because, until now, the angular focusings of the divergent ion beam, which are actually very desirable, have not been satisfactorily solved.
It is, however, also known that time-of-flight mass spectrometers exist which incorporate cylindrical capacitors in the flight path, thus enabling a small instrument to have a long flight path. In this case, a cylindrical capacitor offers angular focusing (for the angle xcfx86, which lies in a plane which intersects the cylinder axis at right angles), angular focusing with respect to energy spreads and time-of-flight focusing with respect to the initial angular spreads for ions of the same specific mass, which can be used for long flight paths.
J. M. B. Bakker (Int. J. Mass Spectrom. Ion Phys. 6(1971)291-295) presents an instrument which achieves energy spread focusing using a combination of straight flight paths with flight paths in cylindrical capacitors. In this paper, both the angular focusing for xcfx86 and the angular focusing with respect to energy spreads in cylindrical capacitors seem to be known, and it is shown that for purely energy focusing, one can shorten the rotational angle for the energy focusing using a combination of linear and circular paths.xe2x80x94Combinations of linear and circular flight paths for angular focusings have been known for many decades and details can be found in relevant text books.xe2x80x94A. A. Sysoev et al. (Fresenius J. Anal. Chem. 361 (1998) 261-266) present an instrument which incorporates a cylindrical capacitor of 509xc2x0 whose energy dispersion appears to be neutralized again by means of a linear continuation of the path to the detector. The 509xc2x0 are only depicted in a diagram, the precise conditions of the energy focusing are not given.xe2x80x94In an ion-optical paper on time-of-flight mass spectrometers with electric sector fields (cylindrical capacitors), A. A. Sysoev (Eur. J. Mass Spectrom. 6 (2000) 501-513) demonstrates solutions for using shorter circular trajectories in cylindrical capacitors in combination with linear flight paths.
In a cylindrical capacitor, ions which enter monoenergetically in a point undergo angular focusing with respect to the angle of incidence "PHgr" after 127.28xc2x0=180xc2x0/√2; ions of the same specific mass experience thereby a time-of-flight dispersion, however. This focusing means that ions with different starting angles come together again in the trajectory at one focal point, but ions of the same mass do not arrive there simultaneously because the path lengths for the ions of different angles are different. We will call this type of focusing xe2x80x9cangular focusing with time-of-flight dispersionxe2x80x9d.
After sweeping this angle twice, i.e. after sweeping an angle of 254.56xc2x0=2xc3x97127.28xc2x0(360xc2x0/√2), an angular focusing then occurs again, but this time together with a time-of-flight focusing (if a time-of-flight focusing was present at the beginning of the first angle), since the time-of-flight dispersion of the first half is precisely compensated for. We will call this focusing xe2x80x9cangular focusing with time-of-flight focusingxe2x80x9d.
In a cylindrical capacitor, ions which enter in a point and are time-of-flight focused but energy dispersive become spatially focused with respect to their energy spread after sweeping an angle of 254.56xc2x0=2xc3x97127.28 xc2x0=360xc2x0/√2; ions of the same specific mass experience a time-of-flight dispersion as a result, however. This focusing means that ions with different energies of incidence come together again in the trajectory at one focal point, but ions of the same mass do not arrive there simultaneously because the path lengths for the ions with different energies are different. We will call this type of focusing xe2x80x9cenergy focusing with time-of-flight dispersionxe2x80x9d. After this special angle there thus occurs an xe2x80x9cangular focusing with time-of-flight dispersionxe2x80x9d and an xe2x80x9cenergy focusing with time-of-flight dispersionxe2x80x9d.
After sweeping this angle twice, i.e. after sweeping an angle of 509.12xc2x0=2xc3x97254.56xc2x0=360xc2x0xc3x97√2, an energy focusing then occurs again, but unfortunately this time without the time-of-flight focusing which occurs with angular focusing. The time-of-flight dispersions do not compensate each other but double instead. In the case of cylindrical capacitors it is therefore generally not possible to achieve an xe2x80x9cenergy focusing with time-of-flight focusingxe2x80x9d.
The time-of-flight dispersion of the energy focusing after 254.56xc2x0 is worth mentioning because here, the lower energy, i.e. slower, ions fly ahead and the higher energy ions arrive later. It is thus possible to again compensate the energy dispersion with a linear flight path. This flight path is, however, relatively long so that it is not possible to build a particularly small mass spectrometer simply by combining a cylindrical capacitor and a linear flight path.
One approach begins with the idea of positioning two cylindrical capacitors, each with 254.56xc2x0, opposite each other in such a way that the trajectory through both cylindrical capacitors resembles a xe2x80x9cfigure 8xe2x80x9d. In each case, straight flight paths, whose length is determined by the radius of the cylindrical capacitors, are then created between the circular trajectories in the cylindrical capacitors. However, these straight flight paths are unfortunately too short to compensate the time-of-flight dispersion which arises as a result of the sweep through the cylindrical capacitors. A time-of-flight dispersion remains which increases with each repeated sweep through the xe2x80x9c8xe2x80x9d and which can only be compensated by a longer, linear flight path. The longer, linear flight path prevents the construction of a very small instrument.
The invention involves virtually increasing the lengths of the straight flight paths between the two cylindrical capacitors for the ions, in order to compensate the time-of-flight dispersion of the cylindrical capacitor with 254.56xc2x0 by means of this internal flight path. The virtual extension of the linear flight path is caused by a flight path which is at a different potential referred to the mid potential in the cylindrical capacitors. The ions must be decelerated as they emerge from the cylindrical capacitor and accelerated again as they enter the next cylindrical capacitor. The ions therefore fly slower in this flight path and, since the energy spread of the ions remains constant, the faster ions can catch up with the slower ones on a shorter path. With a simple adjustment of the potential of the linear flight path, optimum compensation of the time-of-flight dispersion can be achieved.
Special corrective potentials must be inserted between cylindrical capacitor and straight flight paths in order to achieve a good transition in spite of the deceleration. The corrective potentials are applied to corrective electrodes and consist of one pair of electrodes to compensate for the scattering potential of the cylindrical capacitor and one pair of electrodes which forms an ion lens.
Ions which are parallel and time focused when they enter one of the cylindrical capacitors experience two angular focal points each time they sweep through a cylindrical capacitor and are again parallel each time they emerge. (Other types of operation are also possible and are described below). At the end of each of the linear flight paths (before the ions enter the next cylindrical capacitor) a time-of-flight focusing of ions of the same mass is always achieved.
Therefore, if a pulsed ion source is mounted in such a way that a parallel, time focused entry of the ions into the first cylindrical capacitor is achieved then, at the end of the linear flight path which was swept last, a detector can measure a high resolution mass spectrum. Further possible geometries for the operation are discussed below. In particular, an ion beam can be helically spiraled in each cylindrical capacitor by injecting it at a slightly oblique angle (with a motion component in the direction of the axis of the cylindrical capacitors) so that after multiple sweeps, the ion source and detector do not cause an obstruction.
This invention can be used to construct different configurations of relatively small time-of-flight mass spectrometers; in each case the configuration depends greatly on the type of ion generation and the planned mass resolution. It is particularly worth mentioning, for example, an embodiment for ions of a continuous ion beam in the y-direction parallel to the axial direction of the cylindrical capacitor, from which the ions of individual sections of the ion beam are pulser injected in the form of an ion ribbon in the y-direction tangentially into the cylindrical capacitor. The ions thus accelerated fly obliquely out of the pulser in the form of an ion ribbon, and the initial velocity of the ions in the x-direction is maintained. As already described above, the angle to the y-direction is xcex1=arctan √(ExEy), where Ex is the kinetic energy of the ions in the primary beam in the x-direction and Ey the energy of the ions after being accelerated in the y-direction. When the cylindrical capacitor is correctly dimensioned, this angle xcex1 produces the helical spiraling of the ion trajectory within each cylindrical capacitor.
It is not necessary that the pulser and detector are mounted between the cylindrical capacitors. By moving the two cylindrical capacitors axially with respect to each other, the pulser or detector can also be further away from the entrance into the cylindrical capacitor than the length of the straight path between the two cylindrical capacitors; the ion beam is led past the end of the cylindrical capacitors in each case. The overcompensation of the time-of-flight dispersion by the longer path can thus be compensated by adjusting the potential of the straight flight paths because the time-of-flight compensation is achieved by the sum and does not depend on the time-of-flight compensation of the individual paths.